Compact support system using low aspect ratio composite tensile support bands

ABSTRACT

A support system comprising a number of racetrack-shaped tensile support bands ( 22 ) held in tension between a supported article and a supporting article, respective curved end portions of each tensile support band being retained by respective mounting points ( 30 ) mounted on respective surfaces of the supported article and the supporting article, respectively. At least one of the tensile support bands is arranged so that the curved end portions lie in a plane substantially parallel to the surface upon which the respective mounting point is mounted; and at least one mounting point is a monolithic mounting point having a curved surface ( 32 ) which is complementary to an inner curved surface of an end of the tensile support band.

The present invention relates to support structures comprising tensilesupport bands, particularly such structures for retaining concentricvessels, and more particularly such structures with low thermalconductivity for supporting closely spaced concentric vessels incryogenic applications.

In systems such as cryostats for holding superconducting magnets formagnetic resonance imaging (MRI) or nuclear magnetic resonance (NMR), orfor space-borne dewars, it is necessary to firmly mechanically supportconcentric hollow vessels, in such a way that the support structureconducts very little heat. Conventional arrangements include the use offibre reinforced racetrack-shaped tensile support bands. The term‘racetrack-shaped’ is understood in the art to refer to a loop shapecomprising two parallel sides joined by semicircular end portions. Suchtensile support bands are held by mounting points on the outer and innervessels. The tensile support bands are put under tension and the innervessel is thereby supported within the outer vessel, but not in contactwith it other than through the tensile support bands. By making thetensile support bands of fibre-reinforced composite materials, they maybe made to have a very high tensile strength, and a very low thermalconductivity. Tensile support bands made from composite materialscontaining fibres such as glass or carbon offer unique strength,stiffness, and thermal properties compared with other materials.However, the mounting points that allow the tensile support band to beconnected to the vessels are bulky and complicated relative to thetensile support band itself. The result is that the potential spacesaving advantages of the high strength composite cannot be fullyexploited in many applications, since the minimum separation betweenconcentric vessels becomes defined by the need to accommodate themounting points for tensile support between the concentric vessels. Thehigh strength and stiffness, and advantageous thermal properties ofglass, carbon or other fibre reinforced plastics are widely utilised forload bearing tensile support bands. Such tensile support bands aretypically manufactured using a filament winding process to achieve thebest strength of the fibres, commonly in the form of racetrack shapedtensile support bands. In order to achieve the highest tensile strength,the geometry of the tensile support bands at the loop ends must bemaintained within certain parameters, in particular the diameter tothickness ratio (D/t) must be kept high (typically >10), leading to across section of high aspect ratio (W/t) (typically width to thicknessratio >10). Such tensile support bands are sensitive to bending loads,which result in severely reduced tensile strength. In practicalapplications, where misalignment and movements of the supportedstructures occur, the ends of the tensile support bands must besupported using a suitable, typically spherical, bearing arrangement.

Examples of arrangements using tensile composite tensile support bandsare described by R. Kevin Giesy in Cryogenic EngineeringConference/International Cryogenic Materials Conference July 17-21,1995, Columbus Ohio and by R. P. Reed and M. Golda in Cryogenics 37(1997) pages. 233-250.

FIGS. 1A and 1B illustrate a conventional mounting point 10 for atensile support band. The minimum height H of the mounting point islargely determined by the need to provide a bearing arrangement. Asupport pin 12 is carried by a generally U-shaped support 14, shown cutaway in FIG. 1A. The support 14 is itself rigidly mechanically attached,for example by welding, to one of the vessels 15 between which thetensile support band 20 is to be installed. A bearing 16 of generallyspherical shape is provided on the support pin 12. A support roller 18is provided, having an inner surface complementary to the bearing 16,and an outer surface of cylindrical form. The tensile support band 20itself is placed over, and in contact with, the cylindrical outersurface of the support roller 18. The support roller is free to move,within a limited range, by rotation about the bearing 16. This allowseffective retention of the tensile support band 20 even in the case ofmisalignments and movement, avoiding placing uneven loading on thetensile support band, which might otherwise occur if such a mountingpoint were not provided.

The height H of the mounting point could possibly be reduced by reducingthe loop diameter D of the tensile support band. However, in order topreserve the ratio D/t>10, the thickness t of the tensile support bandwould have to be reduced by the same proportion. This, in turn, wouldrequire an increase in the width W of the tensile support band tomaintain the required tensile support band strength. The reduceddiameter D and increased width W would cause the tensile support band tobe even more susceptible to damage by bending loads due to misalignmentor movement. Changing the orientation of the tensile support band sothat the pin 12 were mounted perpendicular to the surface of the vessel15 would typically not reduce the height H since the width W of thetensile support band is typically greater than its diameter D.

FIG. 2 illustrates a typical conventional tensile support band 20. Thediameter D is reduced as far as possible, to allow the height H of themounting point 10 to be minimised. In order to maintain the preferredratio D/t>10, the thickness t is also minimised. In order to maintain arequired tensile strength despite such reduced thickness, the width W isincreased.

The combination of high aspect ratio W/t cross-section and the need toaccommodate a bearing 16 results in a relatively large and complexsystem in which the size of the tensile support band mounting points 10exceed the rest of the tensile support band 20. In many applications thespace available for installation of tensile support bands and theirassociated mounting points is limited, and minimising the space requiredis an important design requirement, which is thus compromised. This isparticularly applicable to the suspension of close-fitting concentricvessels such as are used in super-conducting magnet cryostats. Inaddition, the cost of the bearing assembly of mounting point 10 isrelatively high.

The present invention provides an arrangement which aims to alleviate atleast some of the drawbacks of the prior art. The present inventionaccordingly provides methods and/or apparatus as defined in the appendedclaims.

The above, and further, objects, advantages and characteristics of thepresent invention will become more apparent by reference to thefollowing description of certain embodiments thereof, in conjunctionwith the accompanying drawings, wherein:

FIGS. 1A and 1B illustrate a tensile support band mounting pointaccording to the prior art;

FIG. 2 illustrates a tensile support band according to the prior art;

FIG. 3 illustrates a tensile support band according to an embodiment ofthe present invention; and

FIG. 4 illustrates a mounting point, according to an aspect of thepresent invention, for a tensile support band according to the presentinvention.

The present invention provides a novel form of tensile support band, andof a mounting point for a tensile support band, which has a very lowprofile, is cheap to produce and is simple to install.

FIG. 3 illustrates a tensile support band 22 according to an aspect ofthe present invention. Contrary to conventional thinking, the tensilesupport band of the present invention maintains the diameter tothickness ratio D/t>10, but significantly reduces the width W of thetensile support band while maintaining tensile support band strength andcross-sectional area by increasing the thickness t. For the purposes ofcomparison, it is assumed that the tensile support band of the inventionillustrated in FIG. 3 is formed of the same material as that of theconventional tensile support band of FIG. 2. The tensile support band ofthe invention may be formed of any material conventional for suchapplications, such as a resin reinforced by fibres such as carbon fibre,glass fibre, Kevlar fibres or alumina fibres.

In the illustrated embodiment, the aspect ratio of width W to thicknesst is approximately 1.0. In the tensile support band 22 of the presentinvention, the dimensions are changed to a much lower aspect ratio thannormal, such that the width W is reduced to approximately equal thethickness t, which thickness is itself increased to maintain a requiredtensile strength. In order to preserve strength the loop diameter is inturn increased to maintain the D/t ratio in the favourable region (>10).Although this diameter D is the larger than for a conventional tensilesupport band, the present invention provides a novel mounting point andmethod for installing tensile support bands, which allow installation ina small gap, for example between concentric vessels. By virtue of itsgeometry, the low aspect ratio tensile support band of the invention(FIG. 3) is inherently less sensitive to bending loads than theconventional high-aspect ratio tensile support band (FIG. 2).

According to an aspect of the present invention, the bearing-basedmounting point arrangement 10 of the prior art (FIGS. 1A, 1B) may bereplaced with a simple fixed mounting point in the form of a hook. Theresultant tensile support band and mounting point arrangement is bothcompact and simple, and readily adaptable to economic installation inconfined spaces between, for example, concentric vessels. FIGS. 4A-4Cshow a mounting point 30 according to an aspect of the presentinvention. The mounting point 30 is a monolithic mounting point having acurved surface 32 which is complementary to an inner curved surface ofan end of the tensile support band. 22. Preferably, it is in the form ofa simple hook having an overhanging retaining portion 34, provided toretain the tensile support band 22 and to prevent it from slipping offof the curved surface 32. A protruding lower lip 36 is preferably alsoprovided. This may assist in retaining the tensile support band in itsintended position, and also prevents the tensile support band fromcoming into direct contact with the vessel 15.

The mounting point 30 is preferably attached directly to one of thevessels 15 between which the tensile support band is to be installed,for example by welding or bolting. Unlike the arrangement of the priorart shown in FIGS. 1A-1B, the tensile support band is arranged so thatthe curved end portions of the racetrack shaped tensile support band liein a plane substantially parallel to the surface of the vessel 15 uponwhich it is mounted. That is, the tensile support band is positioned ina position perpendicular to the position of the tensile support bandshown in FIGS. 1A, 1B.

To attach the tensile support band 22 to the mounting point 30, one needonly hook the tensile support band over the mounting point and pull thetensile support band back so that the inner surface of one curved end ofthe tensile support band is in contact with the complementary curvedsurface 32 of the mounting point. Preferably, the overhanging retainingportion 34 is not present over portions 38 of the mounting point spacedfurther apart than the loop diameter D of the tensile support band. Thisis to facilitate installing of the tensile support band over themounting point, by allowing the tensile support band 20 to be installedto the mounting point 30 without flexing the tensile support band. Theabsence of the overhanging retaining portion in this location has notbeen found to cause any difficulties. This feature facilitatesinstallation of the tensile support band in confined spaces betweenvessels where access is limited. The present invention also assists ininstallation of tensile support bands where the mounting point is out ofreach and out of sight.

Although the loop diameter D is large, compared to the conventionalarrangements of FIGS. 1A-1B the low tensile support band width (5.9 mmin a certain example described below) allows use of a mounting point 30of a very compact overall height H−in this example about 12 mm. Thisrepresents a 60% reduction in mounting point height H over theconventional tensile support band example of FIGS. 1A, 1B. By arrangingthe tensile support band of the invention in the orientation describedwith reference to FIGS. 4A-4C, a lower profile tensile support band andmounting point arrangement is provided. This allows installation in moreconfined spaces, for example, more closely packed concentric vessels,than the prior art arrangement of FIGS. 1A-1B.

In many applications, for example the concentric vessels used insuperconducting magnet cryostats, the large loop diameter is not anissue, but the reduced height H translates directly into a reduction inthe space required for the suspension system. A further advantage of thelow height is a significant reduction in the bending moment applied tothe surface of the vessel as a result of the suspension load, withconsequent reduction in the local strength requirement of the vessel.

To allow for misalignment and vessel movements, the overhangingretaining portion 34 and the protruding lip 36 may be tapered—forexample an included angle α of 6° to allow ±3° of movement. Inalternative embodiment, the overhanging retaining portion 34 and theprotruding lip 36 may be spaced apart by a distance greater than thewidth of the tensile support band 22, again to allow a similar degree ofmovement. Despite the low aspect ratio W/t of the tensile support band22 of the invention, misalignment still results in some minor increasein edge stresses in the tensile support band at points of contact withthe mounting point 30. An optional refinement of the mounting point 30to reduce this effect is the use of a compliant, low friction layerinterposed between the tensile support band 22 and the curved surface 32of the mounting point. For example, a reinforced PTFE tape may be usedas the compliant, low friction layer. Alternatively, a low-modulusreinforcing layer, for example woven GRP, may be placed on the innercurved surface of the tensile support band; or different cross-sectionprofiles may be provided at the contacting surfaces of the tensilesupport band 22 and the mounting point—for example “barrelling” of oneor other or both of these surfaces to reduce edge stresses.

A complete summary of the dimensions of an example low aspect ratiotensile support band 22 of the present invention are provided in Table1, for comparison with the corresponding dimensions of a typicalconventional tensile support band 20 of similar strength. The tensilesupport bands are illustrated in FIG. 2 (conventional tensile supportband 20) and FIG. 3 (low aspect ratio tensile support band 22). TABLE 1low aspect conventional ratio tensile tensile support band supportParameter symbol 20 band 22 Loop diameter D 20.5 mm 69 mm thickness t1.76 mm 5.93 mm width W 20 mm 5.93 mm length L 252 mm 252 mmdiameter/thickness ratio D/t 11.7 11.6  cross section area (bothstrands) csa 70.4 mm² 70.3 mm² width/thickness ratio W/t 11.4 1.0length/loop diameter ratio L/D 12.3 3.7 typical mounting point height H30 mm 12 mm

To allow a fair comparison, the key parameters of total cross sectionarea of both strands csa and length L are held constant for both tensilesupport bands. In addition, to maintain strength, the parameter D/t isheld approximately constant for both tensile support bands at a leveltypical of high performance tensile support bands (11.7 in thisexample). The key feature of the new design is the width/thickness ratioW/t, or aspect ratio, which is reduced from he value of 11.4 in thisexample to 1.0. A typical range of values for the ratio W/t is 10-20 forconventional tensile support bands.

As discussed in relation to FIGS. 1A and 1B, the conventional tensilesupport band 20 (FIG. 2) typically uses a pin 12, with a sphericalbearing 16 or other equivalent arrangement to allow for misalignment andensure that the tensile support band 20 is subjected only to puretensile loads. In the example shown in FIGS. 1A, 1B, the minimum heightH of the termination above the surface of the suspended vessel 15 isapproximately 30 mm. Alternative arrangements are possible—for examplerotating the tensile support band through 90°—but may result inincreased height H due to the approximately equal tensile support bandwidth/loop diameter (in the illustrated example, W=20 mm while D=20.5mm).

The present invention provides a tensile support band and mounting pointthat allows the use of a compact and simple connection system withoutlosing the strength of the tensile support band. Compared withconventional designs the cross-section of the tensile support band has alow aspect ratio W/t, is insensitive to bending loads, and allows theelimination of the conventional bearing arrangements.

The primary advantages of the tensile support band and the mountingpoint arrangement of the present invention are:—

-   -   Substantially reduced height required for location of mounting        points, which is especially useful in applications where        concentric vessels are supported inside each other and a        separation distance between the vessels should be minimised.    -   Simplified mounting points may be provided, with consequent cost        advantages.    -   The reduced height of the mounting points directly causes a        reduced height in the point of application of the tensile strain        onto the vessel, reducing stresses in the end connections and        associated vessels, in turn allowing simplification of these        parts.

While the present invention has been described in relation to a limitednumber of embodiments, given by way of examples only, variousmodifications and amendments may be made within the scope of theinvention as defined by the appended claims.

For example, while the example tensile support band has been describedas having an aspect ratio of about 1.0, the invention may be applied toother tensile support bands having a relatively low aspect ratio,typically in the range 0.5-5.0. However, a preferred range of aspectratio W/t is 0.7-1.5. While the invention has been described in terms ofmounting arrangements for cryogenic vessels, the present invention findsapplication in any equipment where tensile support bands are in use.

1. A support system comprising a number of racetrack-shaped tensilesupport bands held in tension between a supported article and asupporting article, respective curved end portions of each tensilesupport band being retained by respective mounting points mounted on asurface of the supported article, and a surface of the supportingarticle, respectively, characterised in that: at least one of thetensile support bands is arranged so that the curved end portions lie ina plane substantially parallel to the surface upon which the respectivemounting point is mounted; and at least one mounting point is amonolithic mounting point having a curved surface which is complementaryto an inner curved surface of an end of the tensile support band.
 2. Asupport system according to claim 1 wherein the mounting point furthercomprises an overhanging retaining portion.
 3. A support systemaccording to claim 2 wherein the overhanging retaining portion is notpresent over portions of the mounting point spaced further apart thanthe inner diameter of the curved end of the tensile support band.
 4. Asupport system according to claim 2 wherein the at least one mountingpoint further comprises a protruding lower lip for preventing thetensile support band from coming into direct contact with the surfaceupon which the respective mounting point is mounted.
 5. A support systemaccording to claim 1, wherein the width/thickness aspect ratio of thetensile support band lies in the range 0.5 to 5.0.
 6. A support systemaccording to claim 1, wherein the width/thickness aspect ratio of thetensile support band lies in the range 0.7 to 1.5.
 7. A support systemaccording to claim 1, wherein the width/thickness aspect ratio of thetensile support band is approximately 1.0.
 8. A method of supporting anarticle to be supported from a supporting article using a systemcomprising a number of racetrack-shaped tensile support bands held intension between respective mounting points mounted on a surface of thearticle to be supported, and a surface of the supporting article,respectively, wherein at least one mounting point is a monolithicmounting point having a curved surface which is complementary to aninner curved surface of an end of the tensile support band, and themethod comprises arranging at least one of the tensile support bands sothat at least one of its curved end portions lie in a planesubstantially parallel to the surface upon which the respective mountingpoint is mounted.
 9. A method according to claim 8, further comprisingthe step of attaching the tensile support band to one of the mountingpoints by hooking the tensile support band over the mounting point andpulling the tensile support band back so that the inner of the curvedend of the tensile support band is in contact with the curved surface ofthe mounting point.
 10. A method according to claim 8 wherein thewidth/thickness aspect ratio of the tensile support band lies in therange 0.5 to 5.0.
 11. A method according to claim 8 wherein thewidth/thickness aspect ratio of the tensile support band lies in therange 0.7 to 1.5.
 12. A method according to claim 8 wherein thewidth/thickness aspect ratio of the tensile support band isapproximately 1.0.
 13. (canceled)